October 10 – Locking away Carbon inside Concrete is an idea that is beginning to gain traction in the often conservative world of construction. A host of startups are experimenting with different ways of using CO2 in the production of concrete, from injecting it into the mix to growing synthetic aggregates.

More than 4 billion metric tons of cement, concrete’s crucial binding agent, are produced each year, with chemical reactions and, to a lesser extent, energy use in its manufacture accounting for as much as 8% of all global emissions.

Alternatives to concrete, such as timber and steel, have their place, but can never fully replace the world’s most ubiquitous man-made substance. “Concrete is quite a beast to tackle,” says Frances Yang, leader of the sustainable materials practice in Arup’s Americas region.

But alongside looking to decarbonise the way it is produced, the sector is also starting to explore the role that our built environment can play as a carbon sink. “Building products are durable and long-lasting … (and) they present an opportunity to store carbon for longer,” adds Yang.

One of the first companies to commercialise a technology for doing this is Canadian firm CarbonCure, which has developed a process of mineralisation in which CO2 is injected into the concrete mix, transforming the gas into a mineral called calcium carbonate. This then reduces the amount of cement that needs to be added, while maintaining the concrete’s strength. “Same performance, same prices but with a lower (carbon) footprint,” says CarbonCure’s chief executive Rob Niven.

So far, the technology has been retrofitted to 800 concrete plants across 34 countries (globally there are 125,000 such plants), with demand not just in developed markets but, crucially, in developing economies too, says Niven. “That’s where all the concrete is being poured and that’s where the CO2 emission reductions and removals need to happen,” he explains. “Concrete naturally pulls you into a lot of these emerging markets, which tend be starved of climate solutions.”

CarbonCure largely relies on using CO2 that has been captured from big emitters, such as ethanol plants, which is delivered to sites by a third party. However, working with San Fransico’s Heirloom Technologies, the company is developing an atmospheric supply of CO2 by combining Direct Air Capture (DAC) with concrete production for the first time.

Heirloom heats crushed limestone, a widely available and inexpensive material, to release naturally absorbed CO2, leaving behind calcium oxide rock. Laid out on huge trays, the rock then acts like a sponge, pulling CO2 from the air, with the potential to soak up to half its weight in the gas in just three days. The rock is then reheated, and the process begins again, with the captured CO2 gas either permanently stored underground or, in this case, injected into concrete.

In this first trial, the numbers were small, with around 30kg of CO2 collected, but as Julio Friedmann, chief scientist at carbon management consultancy Carbon Direct, told Reuters, the potential is huge. “The thing about direct air capture and concrete is it’s a big prize together.”

Indeed, Heirloom recently signed one of the largest carbon dioxide removals deals to date: a $200 million long-term contract with Microsoft to purchase up to 315,000 metric tons of CO2 removal. Microsoft’s director of energy and carbon, Brian Marrs, said in a press release: “We believe that Heirloom’s technical approach and plan are designed for rapid iteration to help drive down the cost of large-scale direct air capture at the urgent pace needed to meet the goals of the Paris Agreement.”

CarbonCure’s Niven says the idea of finding a new use for captured CO2, rather than simply pumping it underground, is important, and the company has even found ways to inject the gas into the wastewater used to clean out trucks. The CO2 mineralises as soon as it comes into contact with any cement material suspended in the water, which can then be used to make fresh concrete.

Niven has also set out to end the so-called green premium for environmentally friendly products, by providing and installing equipment free of charge. If you want to encourage take-up of the technology, he says, you can’t expect concrete producers to be out of pocket. “There’s no margin in this industry, it’s razor thin,” he says. “They don’t have the luxury, like tech companies, to pay a lot of money for climate solutions.”

CarbonCure makes a return by selling the carbon removal credits the technology creates, although the cost varies according to the client, which include Shopify, Stripe and IT infrastructure specialist Hut 8. Industry estimates point to a figure of $321 per ton of carbon removed through mineralisation.

In July the company announced details of its latest funding round, which brought in investments of over $80 million, with investors including global investment fund Blue Earth Capital , Amazon’s Climate Pledge Fund and the Microsoft Climate Innovation Fund.

California-based CarbonBuilt has focused its attention on the humble concrete block, over 1.5 billion of which are produced in the U.S. each year. Commercial production is now under way at a plant in Alabama, with the emphasis on replacing the amount of expensive, high carbon Portland cement needed in the mix with alternatives derived from low-cost industrial byproducts. These materials harden by chemically reacting with CO2 that is piped into the curing chamber from a furnace fuelled by waste biomass.

CarbonBuilt, which also has a carbon removal purchase agreement with Shopify, is also working on using direct air capture (DAC), explains chief executive Rahul Shendure, reducing the need to burn fuel, although the company stresses it only uses biomass that would otherwise be left to rot and produce CO2.

Like Niven, Shendure believes there are profitable uses for captured carbon and that it should be treated as a valuable material in its own right. This, he believes, will help to drive down the cost of DAC and accelerate atmospheric carbon removal.

There is more cutting-edge science from California’s Blue Planet Systems, which mixes CO2 captured from heavy industrial emitters with a calcium source such as recycled concrete, to create a synthetic limestone aggregate. This can then be combined with cement to produce more sustainable concrete, which, says the company, can permanently lock away up to 440 kg of CO2 in every metric ton of aggregate produced.

Arup’s Yang says that with aggregate making up as much as 80% of concrete, it makes sense to focus on sequestering as much carbon as possible into this element of the mix.

Elsewhere, building materials manufacturer Holcim is looking at ways to re-carbonate recycled concrete to create new building materials. It’s FastCarb technology again pulls on CO2 from other industrial sources, with the concrete rubble absorbing the CO2 as part of the process. The carbon is then locked away inside whatever product the concrete is used to make.

These technologies also give rubble a new, practical use, says Tim Riley, chief executive of the World Cement Association, and reduces the use of virgin raw materials. He believes it represents an opportunity to do more with reclaimed concrete. “At the moment we’re not recycling concrete; what we do is break it up … and it ends up in a hole in the ground,” he says.

Niven says that while over time buildings, roads and bridges may be demolished, the limestone within concrete that has been created by its reaction with CO2 is a stable material. “A building may come and go,” he explains, “but the CO2 will always be locked in … It will never get released again and that’s a key benefit of using concrete over timber, which will decompose and re-release the CO2. This is a solution with permanence.”

Despite the proliferation of companies exploring concrete’s potential to sequester CO2, the technology is not yet widely available, partly due to the fact that the industry has set ideas around the make-up of a mix and how much needs to be poured for certain jobs, says Yang. “(We are) trying to move away from that now to enable and support the use of innovative technologies and get to lower carbon concrete by moving towards performance-based specifications.”

Niven’s goal is to double the number of plants that CarbonCure retrofits each year, and he believes governments, which buy around 40% of the concrete globally, can do more to support the sector. They need to specify carbon-busting materials, he says, and help create “an advantage for concrete producers that are showing leadership”.

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